1. Field of the Invention
The present invention relates to a rear wheel steering apparatus.
2. Description of Related Art
There is a vehicle of a so-called 4WS (four wheel steering) in which the rear wheels are steered together with the front wheels.
In the four wheel steering, the rear wheel steering mechanism may be divided into two types, a mechanical type in which the rear wheel steering mechanism is mechanically connected to a front wheel steering mechanism as disclosed in Japanese Utility Model Publication (kokai) No. 25,275/1987 and an electromagnetic type in which the rear wheels are steered by a driving force of an electromagnetic drive means such as, for example, an electric motor, in association with the rear wheel steering mechanism. In the rear wheel steering mechanism of the electromagnetic type, a deceleration mechanism is interposed between the drive means and the rear wheel steering mechanism to minimize a capacity of the drive means as small as possible, as shown in Japanese Utility Model Publication (kokai) No. 25,277/987.
The rear wheel steering mechanism of the electric type is such that the rear wheels are exclusively steered electrically so that a sufficient attention should be paid to a fail safe measure against a trouble with a control system. As disclosed in Japanese Patent Publication (kokai) No. 202,977/1986, a neutral holding means is provided on the rear wheel steering mechanism in order to constantly urge the rear wheel steering mechanism toward a neutral position. This arrangement is based on the concept that, if a trouble would occur in a control system, a control over the rear wheel steering is suspended and the rear wheels are forcibly returned to the neutral position.
In the electrical control by the motor over the rear wheel steering, an accuracy of the control is the issue. If a drive control system of the motor including the motor itself would become defective, the rear wheels could not be steered at a desired angle and it is dangerous to drive the vehicle in such a situation.
It is the matter of course that a position at which the motor is rotated should be accurately determined when the rear wheels are steered by the motor. A sensor is separately mounted for sensing the position of the motor rotated and the drive control over the motor is carried out while monitoring the rotational position of the motor through the sensor. If there would be a discrepancy between the actual rotational position of the motor and the output of the sensor, an accurate control over the rear wheel steering could not be made.